The mechanisms by which organisms alter their growth and development in response to changes in their ambient environment are largely unknown. Plants exhibit an enormous array of phenotypic plasticity because most plant organs do not arise until after the seed germinates, allowing organ size and shape to be optimized to the local environment. Because they are sessile and photosynthetic, plants are especially attuned to their light environment. Light influences every developmental transition from seed germination to flowering, having particularly dramatic effects on the morphogenesis of seedlings. Light signals do not act autonomously, but are integrated with seasonal and diurnal changes in temperature, as well as with intrinsic developmental programs to specify correct spatial and temporal regulation of gene expression, organelle development, and cellular differentiation. The proposed studies aim to understand how one photoreceptor, phytochrome B (PHYB), influences the development of plants at various stages of the life cycle. In response to light, phytochromes partition from the cytoplasm to discrete sites in the nucleus, where they initiate a signaling cascade that alters the expression of more than a thousand genes. Genetic and biochemical approaches have identified a number of proteins that act in close proximity to PHYB under a variety of different growth conditions, yet mechanistic details are lacking. The proposed studies will explore the molecular mechanisms of PHYB signaling by characterizing these proteins in terms of their roles in signaling, PHYB trafficking, enzyme activation, or light-regulated gene expression. The primary goals are to: (1) characterize previously identified proteins involved in early events of PHYB signaling that link its regulated nucleocytoplasmic partitioning to phosphorylation, signaling, and transcription;(2) link the detection of shade light by PHYB to the activation of enzymes involved in localized auxin production and changes in body plan;(3) perform genetic screens to define auxin-independent components in shade avoidance. The diverse responses that plants have to light provide a unique model system for understanding phenotypic plasticity. As a result, the study of light signaling in plants has not only provided insight into plant growth and development, but has also led to the discovery of conserved proteins that regulate DNA damage, transcription, and lipid metabolism in metazoans. Project Narrative The diverse and dramatic responses that plants have to light provide a unique model system for understanding how all organisms alter their growth and development in response to changes in their local environment. The system is easy to manipulate, which has resulted in the discovery of key regulatory proteins that are conserved between plants and mammals. The experiments proposed in this study should thus contribute significantly to analyses of complex signal transduction networks in multiple organisms.